A device having energy that can be potentially harmful to a user or which might damage the device should employ an interlock that enables the safe utilization of the device by the user. In those instances where life safety is an issue, a keyed switch is often used as an interlock to guard against exposing the user to potentially hazardous forces or actions. The provision of an interlock serves to put the user on notice that a particular device is associated with a potentially harmful energy, since the interlock must be successfully engaged before the user can interact with the device. For example, an electrical enclosure having a keyed switch (interlock) is often employed to prevent a user from contacting a dangerous enclosed energy, such as a high voltage. The keyed switch causes devices contained within the electrical enclosure to be de-energized when the user opens the enclosure to access its interior. Although interlocks are often employed to de-energize devices having potentially life threatening energies, they are also used to enable a hazardous device to be energized. For example, a user must be typically seated on a lawn tractor to engage an interlock (keyed switch) under the seat that is actuated by the weight of the user, in order to operate the power take-off drive for the mower blades.
Interlock switches that must be continually actuated by a user to enable a device to operate are commonly referred to as "deadmnan" switches. This term refers to the concept that when the user's actuation of the switch goes slack or "dead," then the interlocked device should be de-energized. Typically, a deadman switch is provided on devices that are firmly grasped by the operator and if released, can independently produce a rapid or sudden movement that may be unsafe to the operator. For example, a hand-held chain saw may include a deadman switch that must be engaged (compressed by the user's grip) before the chain can be driven. If the grip of the operator loosens on the handle, the deadman switch disengages, stopping the chain.
Although a haptic force feedback joystick is not as dangerous as a chain saw, if the control handle is not firmly gripped when the force is applied to the control handle, it can potentially injure a user, possibly causing bruises, or worse. In addition, since the control handle of a force feedback joystick is intended to be held by the user while the force is applied, the drive mechanism and control handle may be damaged if a substantial force is applied while the handle is not grasped. In the prior art, mechanical and/or retro-reflective switches have been employed to stop the movement of a force feedback joystick handle when the user is not gripping the handle. A retro-reflective switch employs a light source that produces a light beam, which is reflected from a user's hand applied to grip the control handle so that the reflected light actuates a light sensor. These prior art solutions have several problems. First, mechanical switches typically require periodic maintenance to prevent failure due to oxidation or breakage of their electrical contacts. Second, mechanical switches having a high duty cycle rating are relatively expensive. Third, retro-reflective switches do not work reliably when the light beam of the switch is broken by a non-reflective material, such as a darkly colored glove or a user's hand that has a dark skin tone. Fourth, ambient light sources can easily cause false triggering of retro-reflective switches. Moreover, the prior art teaches a simple interlock that allows the full magnitude of the force to be applied to the control handle as soon as the user grips the handle. If the user's grip has not yet tightened, the application of maximum force can literally tear the control handle loose from the user, so that it causes injury. Significantly, the prior art does not teach slowly increasing the magnitude of the force when the user initially grips the joystick's handle.
Ideally, a user would prefer that the force be applied slowly as the grip on the control handle is set, but any force being applied should immediately subside when the user loosens a grip on the handle. Further, any sensor applied to detect the user's grip on the control handle should be insensitive to the reflectivity of the user's hand or glove worn on the hand and should also be insensitive to ambient light variation. Currently, none of the prior art interlock or deadman switches for a haptic force feedback joystick addresses these problems. It would also be desirable to employ the technique used to disable force applied to the control handle for safety reasons to interrupt force applied to the control handle when the user elects to pause the execution of an application controlling the computer. If a user presses a keyboard button or other control designated to pause a game or other application running on a host computer, the application of force to the haptic joystick control handle would then be disabled just as if the user had released the control handle. Alternatively, by sensing the user's grip on the joystick control handle, it would be possible to automatically pause the game if the user releases the control handle. Thus, in addition to immediately disabling the application of force applied to the control handle in response to this condition, the application or operating system could be programmed to respond by pausing the application or by causing the host computer to enter a "sleep" mode. None of these features related to pausing an application is provided by any prior art haptic control device.